My heart sank when I saw these words: Firefox user interface guru. And yes, he's had an idea. A suggestion: rather than a fancy new UI, how about having a crack at stability? FF 3, and the later FF 2s, were and are crashy, hangy, and inconsistent. It regularly (daily) gets its knickers in a twist and either fails to blit the screen, hangs, reads from the keyboard buffer extremely slowly, or just crashes without error messages, warnings, logs or anything else. And the "Save and Quit" function doesn't work, which is probably connected with the fact that most crashes at least let you restore the tabs, but some lose even that.
If they want a new idea, what about having a crack at whatever is to Firefox as Firefox was to Mozilla, a lightweight, fast, rugged cut-down version of the bloated original? They could keep only the rendering engine and things like SSL, and make everything else an extension. Personally, I'd use Konqueror if it had equivalents for the various extensions I use. Anyway, Mozilla thinks Firefox is an operating system. And the thing about operating systems is that stability, security, and affordances for applications are the first and indeed only things that matter. Fancy user interfaces can be applied later.